The early volumes of GCS (Die griechischen christlichen Schriftsteller der ersten drei Jahrhunderte) are now in the public domain, and many of them have been digitized. There’s a helpful list (probably not up to date) here:

Field’s standard editions of Chrysostom’s commentaries on Paul would also be desirable. (Field’s Chrysostom Matthew appeared early enough to be reprinted by Migne, but unfortunately the Paul vols were just too late for inclusion there--Migne had to rely on greatly inferior 18th century sources instead.)

I know that all the other vols are somewhere on Google Books, because I’ve successfully downloaded them from there, but I can no longer find them. All I remember is that I had to search in some absolutely counterintuitive way (without using any of the words Chrysostom, Chrysostomus, Chrysostomi, or Field). It would probably be easier for USA users. (I’m in Australia, so a lot of Google Books pages are inaccessible to me.)

Even though it's readily available elsewhere, this seems to me the sort of thing that ought to be at Perseus. (Migne's texts of the Apostolic Fathers are, of course, greatly outdated--there were enormous discoveries in that field in the late 19th century!)

When I was studying patristic theology, I usually used the Sources Chrétiennes as a basis for study rather than the editions of Migne. They present the latest in schoarly redaction and present a "convenient" French translation. You might like to consider following those texts for digitalisation.